Aluminum sacrifical anode

ABSTRACT

An aluminum base alloy comprising about 0.01 to about 0.2 weight percent mercury, about 0.1 to about 20 weight percent zinc, and a heavy metal. The heavy metal can be about 0.03 to about 2.0 weight percent bismuth, about 0.001 to about 0.05 weight percent cadmium, and about 0.001 to about 0.04 weight percent silver. Methods of producing the alloy and of using the alloy as a sacrificial anode are described.

Umted States Patent 11 1 [111 3,721,618

Reding et a]. 1March 20, 1973 [54] ALUMINUM SACRIFICIAL ANODE [56]References Cited [75] Inventors: John T. Reding, Lake Jackson; UNITEDSTATES TE S David W. Barnett, Clute, both of Tex 3,415,305 12/1968Schrieber et a1. ..75/146 1 Assigneei l Dow Chemical Company, PrimaryExaminer-Richard 0. Dean Mldland, Mlch- Attorney-Griswold & Burdick, L.S. Jowanovitz and 22 Filed: March 11, 1971 Grace [21] Appl. No.: 123,28457 ABSTRACT An aluminum base alloy comprising about 0.01 to [52] US. Cl...204/ 197, 75/146, 204/148, bout 0,2 weight percent mercury, about 0.1to about 0 /293 20 weight percent zinc, and a heavy metal. The heavy[51] Int. Cl. ..C23f 13/00 m t l can be about 0.03 to about 2.0 weightpercent Field of Search bismuth, about 0.001 to about 0.05 weightpercent cadmium, and about 0.001 to about 0.04 weight percent silver.Methods of producing the alloy and of using the alloy as a sacrificialanode are'described.

14 Claims, 1 Drawing Figure CORROJ/ON M/ /00% HUMIDITY ENVIRONMENT 0FHLUM/NUM 5% ZINC ALLOY ALUMINUM SACRIFICIAL ANODE BACKGROUND OF THEINVENTION This invention relates to sacrificial galvanic anodes and moreparticularly to a novel aluminum base alloy for galvanic anodes having ahigh resistance to air oxidation, a high oxidation potential, and auseful high electrical output per unit mass of metal consumed, that is,a high electrochemical equivalent.

Aluminum sacrificial anodes, containing elements such as mercury andzinc, can be successfully employed to cathodically protect metalstructures, especially steel and other ferrous base metals, fromcorrosion in corrosive environments. Generally, the effectiveness ofaluminum-mercury-zinc alloys for cathodic protection of metals in lowchloride containing environments is minimal. This is believed to beprimarily a result of the voltage of an aluminum-mercury-zinc alloybeing at least partially dependent upon the chloride concentration inthe electrolyte. That is, as the percentage of chlorine ion present inthe electrolyte is reduced the voltage of the anode diminishes.

A sacrificial anode having a voltage in excess of about 0.9 volts withrespect to a saturated potassium chloridecalomel reference electrode isusually desired, since the anode area can be reduced as the voltageincreases. The utility of commonly used commercial aluminum-mercury-zincanode alloys is generally negligible when the chlorine ion concentrationin the electrolyte is lower than approximately 0.25 percent because thevoltage approaches 0.9 volts. It is well known that increasing themercury content in this ternary alloy system will result in an increasein the voltage; however, the higher mercury content usually results inmore rapid air oxidation of the alloy. In fact, sufficient oxidation tocause a significant loss in anode weight can occur before the metal iseffectively utilized as a sacrificial anode.

It is an object of this invention to provide an oxidation resistantaluminum alloy suitable for use as a sacrificial anode.

It is another object of this invention to provide an oxidation resistantaluminum alloy having a voltage of at least 0.9 in electrolytes having achlorine ion concentration less than about 0.25 percent.

These and other objects and advantages will become apparent during thecourse of the following description of the invention.

SUMMARY OF THE INVENTION Thepresent invention comprises a novel aluminumbase alloy composition containing small amounts of mercury, zinc, and aheavy metal. The invention also pertains to galvanic anodes preparedfrom said alloy.

More particularly, the present aluminum alloy composition consistsessentially of from about 0.01 to about 0.2 weight percent mercury,about 0.1 to about weight percent zinc, and the heavy metal of bismuth,cadmium, or silver. Even more specifically, the heavy metal can be about0.03 to about 2.0 weight percent bismuth, about 0.001 to about 0.05weight percent cadmium, or about 0.001 to about 0.04 weight percentsilver.

The alloy preferably consists essentially of about 0.08 to about 0.15weight percent mercury, about 0.4 to about 14 weight percent zinc, and aheavy metal. Suitable heavy metals are about 0.l to about 0.5 weightpercent bismuth, about 0.005 to about 0.02 weight percent cadmium, andabout 0.005 to about 0.02 weight percent silver.

An aluminum alloy sacrificial anode can be produced by melting aluminumby means known to them skilled in the art. Preferably the aluminum has apurity of 99.5 weight percent; and more preferably it has a purity of99.8 weight percent aluminum. Mercury, zinc, and a heavy metal can beadded to the aluminum as elements and/or alloys before, simultaneously,or subsequently to melting the aluminum. Prior to casting the moltenaluminum alloy into the desired final shape, the molten bath should bestirred sufficiently to distribute the aforementioned alloying additionswithin the pure aluminum. Sufficient stirring is characterized by a bathhavinga substantially uniform composition throughout. Casting of themolten bath can be carried out in accordance with procedures known tothose skilled in the art, for example, pouring the molten aluminum alloyinto a mold of predetermined shape and/or by cladding a substrate withthe alloy of this invention. The recited method for producingsacrificial anodes can effectively use the aforementioned alloys.

The claimed alloy can be employed as a sacrificial anode using methodsknown to those skilled in the art. For example, attaching the anode to amore electropositive-metal structure, such as steel ship hulls orboilers, to afford an'electrical contact between the anode and steelcauses preferential corrosion of the anode in corrosive environments.

As can be seen in the following examples, table and appended figure,addition of bismuth, cadmium, or

silver to an aluminum-mercury-zinc alloy produces a sacrificial anodehaving an acceptable resistance to air oxidation, a high useful voltageand a high current capacity (amp-hour per pound output) in corrosiveenvironments having low concentrations of chlorine ion.

EXAMPLES l-l0 Aluminum having a purity of 99.99 weight percent wasmelted in a graphite crucible using a resistance heater as the heatsource. When the aluminum had melted and attained a temperature of 700Csufficient mercury and zinc were added to the liquid aluminum bath toachieve a final alloy composition of 0.12 weight percent mercury and5.0-weight percent zinc. The mercury was added to the liquid aluminum asa master alloy containing 10 parts of zinc to each part of mercury.After the alloying constituents had melted the bath was thoroughlystirred to disperse the ingredients uniformly throughout the aluminum.

For comparative purposes, two castings were produced from the moltenaluminum-mercury-zinc alloy by pouring portions of the metal into a X 3%X 4 inches steel mold and solidifying the alloy into anode castings. Theremaining molten metal was reheated to 700C and 0.01 weight percentsilver added to the metal. The molten metal was stirred until the alloycomposition was substantially uniform throughout. Two castings in thisalloy were then poured and solidified in a 54 X 3% X 4 inches mold.

Each solidified. and cooled casting or slab was weighed and positionedin a humidity cabinet for a period of about 20 hours. The humiditycabinet had a continuous stream of steam passing through to promoteoxidation of the sacrificial aluminum anode samples. After thestipulated exposure time had elapsed each casting was removed from thecabinet, washed with water, dried, and weighed. The magnitude of theweight loss indicated the oxidation loss during the test periods, thatis, as the weight loss increased the metal corrosion increased. Theabove procedure was substantially followed for producing and testingaluminum a1- loys having the chemical compositions shown in Table l andin the appended figure.

As can be seen in thefigure, addition of bismuth, cadmium, or silverimproved the alloys resistance to air oxidation. The bismuth additionalso increased the alloy brittleness.

EXAMPLES 1 1-29 Aluminum alloy specimens of the composition specified inTable 1 were tested under simulated commercial conditions forapproximately 30 days. Each five-eights inch diameter by 5 inch longsample was connected in electrical series with a galvanized steel clothcathode having a mesh size of approximately onefourth inch. Theelectrolyte shown in Table 1 and a glass container completed theelectrolytic test cell. Voltage measurements in the direct current cellwere obtained several times during each test. The total ampere hoursutilized during each test was also measured. Upon completion of thetesting cycle, each aluminum anode was cleaned and weighed to ascertainthe weight loss. The current capacities (amp-hour per pound) shown inTable l were calculated from weight loss and ampere hour measurement.

What is claimed is:

1. An aluminum base alloy consisting essentially of about 0.01 to about0.2 weight percent mercury, about 0.1 to about weight percent zinc, anda heavy metal selected from the group consisting of about 0.03 to about2.0 weight percent bismuth, about 0.001 to about 0.05 weight percentcadmium, and about 0.001 to about 0.04 weight percent silver, andbalance aluminum.

2. The alloy of claim 1 wherein the heavy metal is silver.

3. The alloy of claim 1 consisting essentially of about 0.08 to about0.15 weight percent mercury, about 0.4 to about 14 weight percent zinc,and a heavy metal selected from the group consisting of about 0.1 toabout 0.5 weight percent bismuth, about 0.005 to about 0.02 weightpercent cadmium, and about 0.005 to about 0.02 weight percent silver.

4. The alloy of claim 3 wherein the heavy metal is silver.

5. A method comprising:

a. melting an alloy having at least about 99.5 weight percent aluminum;

b. adding to the aluminum sufficient elements to produce a finalcomposition consisting essentially of about 0.01 to about 0.2 weightpercent mercury, about 0.1 to about 20 weight percent zinc, and a heavymetal selected from the group consisting of about 0.03 to about 2.0weight percent bismuth, about 0.001 to about 0.05 weight percentcadmium, and about 0.001 to about 0.04 weight percent silver;

. stirring the molten aluminum to distribute the additions of step (b)in the molten aluminum; and d. casting the molten mixture of step (c).6. The method of claim 5 wherein silver is added to the aluminum.

7. The method of claim 5 wherein the alloy melted 40 contains at leastabout 99.8 weight percent aluminum.

8. The method of claim 5 wherein the elements added to the aluminum aresufficient to produce a final TABLE I.PE RFORMANCES OF ALUMINUM ALLOYSAS SACRIFICIAL ANODES IN VARIOUS ELECTROLYTES Saturated SyntheticComposition, percent Tap water CaSO4 seawater Amp.- Amp.- Amp.-

Example Hg Zn Ag Bi Volts hr./lb. Volts hr./lb. Volts hr./1b.

1 Current density in: Tap waterabout 125 miliiamps/itfl; SaturatedCBSO4-flb0t1t 300 mllliumps/itfl; Seawnter-abont 125 milliamps/l'tJ.

Seawater-about 20 ohm-cm.

composition consisting essentially of about 0.08 to about 0.15 weightpercent mercury, about 0.4 to about 14 weight percent zinc, and a heavymetal selected from the group consisting of about 0.1 to about 0.5weight percent bismuth, about 0.005 to about 0.02 weight percentcadmium, and'labout 0.005 to about 0.02 weight percent silver.

9. The method of claim 8 wherein silver is added to the aluminum.

10. The method of claim 8 wherein the alloy melted contains at leastabout 99.8 weight percent aluminum.

1 l. A sacrificial anode consisting essentially of about 0.01 to about0.2 weight percent mercury, about 0.1 to about weight percent zinc, anda heavy metal selected from the group consisting of about 0.03 to about2.0 weight percent bismuth, about 0.001 to about 0.05 weight percentcadmium, and about 0.001

to about 0.04 weight percent silver and balance aluminum with a voltageof at least 0.9 in electrolytes with a chlorine ion concentration lessthan about 0.25 percent.

12. The anode of claim 1 1 wherein the heavy metal is silver.

13. The anode of claim 11 consisting essentially of about 0.08 to about0.15 weight percent mercury, about 0.4 to about 14 weight percent zinc,and a heavy metal selected from the group consisting of about 0.1 toabout 0.5 weight percent bismuth, about 0.005 to about 0.02 weightpercent cadmium, and about 0.005 to about 0.02 weight percent silver.

14. The anode of claim 13 wherein the heavy metal is silver.

2. The alloy of claim 1 wherein the heavy metal is silver.
 3. The alloyof claim 1 consisting essentially of about 0.08 to about 0.15 weightpercent mercury, about 0.4 to about 14 weight percent zinc, and a heavymetal selected from the group consisting of about 0.1 to about 0.5weight percent bismuth, about 0.005 to about 0.02 weight percentcadmium, and about 0.005 to about 0.02 weight percent silver.
 4. Thealloy of claim 3 wherein the heavy metal is silver.
 5. A methodcomprising: a. melting an alloy having at least about 99.5 weightpercent aluminum; b. adding to the aluminum sufficient elements toproduce a final composition consisting essentially of about 0.01 toabout 0.2 weight percent mercury, about 0.1 to about 20 weight percentzinc, and a heavy metal selected from the group consisting of about 0.03to about 2.0 weight percent bismuth, about 0.001 to about 0.05 weightpercent cadmium, and about 0.001 to about 0.04 weight percent silver; c.stirring the molten aluminum to distribute the additions of step (b) inthe molten aluminum; and d. casting the molten mixture of step (c). 6.The method of claim 5 wherein silver is added to the aluminum.
 7. Themethod of claim 5 wherein the alloy melted contains at least about 99.8weight percent aluminum.
 8. The method of claim 5 wherein the elementsadded to the aluminum are sufficient to produce a final compositionconsisting essentially of about 0.08 to about 0.15 weight percentmercury, about 0.4 to about 14 weight percent zinc, and a heavy metalselected from the group consisting of about 0.1 to about 0.5 weightpercent bismuth, about 0.005 to about 0.02 weight percent cadmium, andabout 0.005 to about 0.02 weight percent silver.
 9. The method of claim8 wherein silver is added to the aluminum.
 10. The method of claim 8wherein the alloy melted contains at least about 99.8 weight percentaluminum.
 11. A sacrificial anode consisting essentially of about 0.01to about 0.2 weight percent mercury, about 0.1 to about 20 weightpercenT zinc, and a heavy metal selected from the group consisting ofabout 0.03 to about 2.0 weight percent bismuth, about 0.001 to about0.05 weight percent cadmium, and about 0.001 to about 0.04 weightpercent silver and balance aluminum with a voltage of at least 0.9 inelectrolytes with a chlorine ion concentration less than about 0.25percent.
 12. The anode of claim 11 wherein the heavy metal is silver.13. The anode of claim 11 consisting essentially of about 0.08 to about0.15 weight percent mercury, about 0.4 to about 14 weight percent zinc,and a heavy metal selected from the group consisting of about 0.1 toabout 0.5 weight percent bismuth, about 0.005 to about 0.02 weightpercent cadmium, and about 0.005 to about 0.02 weight percent silver.14. The anode of claim 13 wherein the heavy metal is silver.